Efficient soluble PTCBI-type non-fullerene acceptor materials for organic solar cells

Single perylene diimide (PDI) used as a non-fullerene acceptor (NFA) in organic solar cells (OSCs) is enticing because of its low cost and excellent stability. To improve the photovoltaic performance, it is vital to narrow the bandgap and regulate the stacking behavior. To address this challenge, we synthesize soluble perylenetetracarboxylic bisbenzimidazole (PTCBI) molecules with a bulky side chain at the bay region, by replacing the widely used “swallow tail” type alkyl chains at the imide position of PDI molecules with a planar benzimidazole structure. Compared with PDI molecules, PTCBI molecules exhibit red-shifted UV–vis absorption spectra with larger extinction coefficient, and one magnitude higher electron mobility. Finally, OSCs based on one soluble PTCBI-type NFA, namely MAS-7, exhibit a champion power conversion efficiency (PCE) of 4.34%, which is significantly higher than that of the corresponding PDI-based OSCs and is the highest PCE of PTCBI-based OSCs reported. These results highlight the potential of soluble PTCBI derivatives as NFAs in OSCs. Graphical abstract Supplementary Information The online version contains supplementary material available at 10.1007/s12200-023-00063-6.


Experiment Details:
Materials: All reagents and solvents, unless otherwise specified, were purchased from commercial sources and were used without further purification.

1) Synthesis of MAS-5
1,6,7,12-tetrachloroperylene tetracarboxylic acid dianhydride (compound 1) (0.5 g, 0.94 mmol) was suspended in 25 mL glacial acetic acid. Then o-phenylenediamine (0.22 g, 2 mmol) was added and the mixture was heated to reflux for 6 hours. After cooling to room temperature, the precipitate was filtered off and rinsed with plenty of water. It was dried in a vacuum oven at 100 °C to obtain compound 3 as an insoluble compound.
The unpurified product compound 3 (100.00 mg, 0.15 mmol), 4-tertbutylphenylboronic acid (528.05 mg, 2.97 mmol), anhydrous sodium carbonate (215.20 mg, 1.56 mmol) and tetrakis(triphenylphosphine)palladium (51.41 mg, 0.045 mmol) were added into a Schlenk tube. The nitrogen was purged three times. A mixed solution of ethylene glycol dimethyl ether (4 mL) and water (1 mL), which had been degassed, was added. The reaction was stirred at 80 °C for 48 h. After cooling to room temperature, the mixture was extracted with CH2Cl2, dried with anhydrous Na2SO4. The crude product was purified by silica gel column chromatograph using dichloromethane/petroleum ether (2:1, v/v) as the eluent to obtain MAS-5.

3) Synthesis of PDI-1
Compound 1 (5 g, 12.47 mmol), imidazole (40 g, 587.53 mmol) and zinc acetate (1.75 g, 9.56 mmol) were added into a 250 mL three-necked flask, and then a condenser tube and a gas guide were connected to the reaction. The temperature is gradually increased until the imidazole is melted and the rotor can be smoothly and continuously stirred. Then, 3-aminopentane (2.56 g 29.31 mmol) was added to the reaction vessel with a syringe, and the temperature was kept and stirred overnight. Then, 2 M hydrochloric acid (120 mL) was added, and the mixture was stirred for 20 min. The crude product was purified by silica gel column chromatograph using dichloromethane as the eluent to obtain Compound 2 (6.08 g, 90%) The product compound 2 (1.00 g, 1.5 mmol), 4-tert-butylphenylboronic acid (2.13 g, 11.97 mmol), anhydrous sodium carbonate (2.17 g, 15.71 mmol) and tetrakis(triphenylphosphine) palladium (432.23 mg, 0.374 mmol) were added into a Schlenk tube. The nitrogen was purged three times. A mixed solution of ethylene glycol dimethyl ether (40 mL) and water (15 mL) which had been degassed, was added. The reaction was stirred at 80 °C for 96 h. After cooling to room temperature, the mixture was extracted with CH2Cl2, dried with anhydrous Na2SO4. The crude product was purified by silica gel column chromatograph using dichloromethane/petroleum ether

4) Synthesis of PDI-2
The synthesis method is the same as PDI-1.

5) Optimized synthesis of MAS-5
PDI-1 (200 mg, 0.19 mmol), KOH (550.8 mg, 9.82 mmol) and tert-butanol (30 mL) were added into a 100 mL round bottom flask, condensed to reflux 6 hours. Then 10 mL of concentrated hydrochloric acid and 10 mL of water were added, then stirring for another 30 min. The mixture gradually changed from a transparent orange solution to a dark green solution, and solid particles appeared at the bottom of the flask. The product was filtered and dried. The crude product was purified by silica gel column chromatograph using dichloromethane/petroleum ether (2:1, v/v) as the eluent to obtain compound 5. (156 mg, 89.7% yield) Compound 5 (150 mg, 0.163 mmol) was suspended in 10 mL glacial acetic acid.
Then o-phenylenediamine (38.8 mg, 0.358 mmol) was added and the mixture was heated to reflux for 6 hours. After cooling to room temperature, the precipitate was filtered off and rinsed with plenty of water. The crude product was purified by silica gel column chromatograph using dichloromethane/petroleum ether (2: EPR spectra were obtained on the Bruker A300 electron paramagnetic resonance (EPR) spectrometer.

2) Optical characterizations.
UV-vis absorption spectra were recorded on a PerkinElmer Lambda 35 spectrophotometer. Molar absorption coefficient test: according to Beer-Lambert Law: A=ɛbc, where A is the absorbance; ɛ is the molar absorption coefficient; b is the thickness of the liquid tank with the unit of cm; c is the solution concentration with the unit of mol L -1 .

3) Electrochemical characterizations.
Cyclic voltammetry (CV) was done on a CHI604E electrochemical workstation with glassy carbon electrode, platinum electrode, and standard Ag/AgNO3 as working electrode, counter electrode, and reference electrode, respectively, in a 0. For the measurement of polymer HOMO energy level, the polymer solution is uniformly spin-coated on ITO glass and used as the working electrode. Then the energy level of the material is measured by the general method, and its LUMO energy level is calculated by its optical band gap.

4) DFT calculations
Gas-phase B3LYP/6-31G (d, p) ground-state equilibrium geometry optimizations were considered within Gaussian 09. To reduce the computational cost, all alkyl substituents were truncated to methyl groups. Molecular dihedral angles were systematically altered to ensure that the optimized geometric lower energy minimum was not missed structure possessed no imaginary frequencies. The resulting structure was characterized through frequency calculations (at the same level of theory).

5) Solar cell fabrication and testing.
The

7) AFM analysis.
The morphologies of active layers were investigated by Park Systems NX-10 highresolution scanning probe microscope. The specimen for AFM measurements was prepared using the same procedures for fabricating devices.

8) TEM analysis.
Transmission electron microscopy (TEM) measurement was performed by using a HITACHI H-7650 electron microscope with an acceleration voltage of 100 kV.      b. Optical band gap is taken from UV-vis absorption onset (Fig. 2b).
c. ELUMO is determined by the relation: ELUMO = EHOMO + Eg opt